If the cross ratio κ of {A B, C D} satisfy κ^2 - κ + 1 = 0, then the points are said to form a bivalent range, and {A B, C D} = {A C, D B} = {A D, B C} = κ {A C, B D} = {A D, B C} = {A B, D C} = - κ^2.

harmonic range